He is very young to be diagnosed with
ADHD; typically that cannot be diagnosed before school
age (i.e., six or seven years of age), because until that time most of the behaviours exhibited by children with ADHD are typical of all children in the preschool years. (They may be more severe in preschoolers who have ADHD, but since some kids who are highly active in their preschool years definitely don't have ADHD, it's better to wait.